This invention relates generally to earth borehole apparatus, and specifically to caliper and centralizing apparatus having borehole wall-engaging faces urged against the borehole wall.
It is well known in the art of well logging to use caliper and centralizing instruments having a plurality of borehole wall-engaging members and having means, for example, a spring, to urge the borehole wall-engaging members against the borehole wall. For example, as discussed hereinafter with regard to the prior art, it has been known to use a bow spring interconnected between two rigid arms to contact the borehole walls. With such instruments, the bow spring generally has to have a large radius and the rigid arms have to be maintained at a relatively low angle of approximately no greater than 20.degree. to 25.degree. to keep the connecting bow spring from overstressing the spring material. With such devices, there is generally a great distance between the pivot points on the rigid arms.
It is therefore a primary object of the present invention to provide a new and improved well bore instrument having means to centralize the instrument within the borehole;
It is also an object of the present invention to provide a new and improved well bore apparatus having means to caliper the earth borehole.
The objects of the present invention are accomplished, generally, by apparatus which includes an elongated support member adapted for movement through an earth borehole, a borehole-engaging member having first and second ends and a centralized pivot point, a first arm member connected between said centralized pivot point and a fixed pivot point on said support member, a second arm member connected between said centralized pivot point and a slidable pivot point on said support member, a third arm member pivotally connected to the said first end of said borehole-engaging member and slidably connected to said first arm member intermediate the ends of said first arm member, and a fourth arm member pivotally connected to the said second end of said borehole-engaging member and slidably connected to said second arm member intermediate the ends of said second arm member.
As an additional feature, a hidden bow spring is attached to the underside of the first and second arm member to maintain the borehole-engaging member flat against the borehole wall.